1. Field of the Invention
The present invention relates to imaging systems and, in particular, to imaging systems that are capable of capturing a sequence of full resolution images at ultra-high framing rates for use in obtaining deformation and/or temperature field measurements of a target under observation.
2. Description of Related Art
High speed imaging systems are widely used in a variety of scientific fields to observe high speed events. Most imaging systems include a light source that illuminates a target, and an image sensor (e.g., a charge coupled device (CCD) array) that captures an image from the target for readout to a memory device. The primary limitation on the maximum framing rates that may be achieved is the time required to readout a captured image to the memory device. Thus, even with the fastest available image sensors, the framing rates of these imaging systems cannot exceed a few kilohertz (kHz) (for high resolution images) due to the limitation on readout time.
In order to achieve higher framing rates, a variety of techniques have been used that rely on reducing the size (i.e., number of pixels) of the image readout to the memory device to thereby decrease the readout time. One technique known as “binning” reduces the size of the image by reading out an averaged value for each neighboring group of pixels on the CCD array (rather than reading out each and every pixel). Another technique known as “windowing” captures a reduced area or window of the field of view of an image (rather than the entire field of view). While the “binning” and “windowing” techniques may be used to achieve framing rates of up to 100 kHz, they do so by decreasing the spatial resolution and/or size of the image.
Another technique that may be used to achieve higher framing rates is similar to the “windowing” technique in that a reduced area of the field of view is used for image capture. However, instead of reading out each image to the memory device immediately after it is captured, multiple images are captured on different areas of the CCD array and then readout together to the memory device. While framing rates of up to 1 megahertz (MHz) may be achieved, this technique also decreases the spatial resolution or size of the images. An example of a camera that employs this technique is the DALSTAR 64k1M™ camera.
Recently, imaging systems have been developed that use a dual-cavity laser to emit two short light pulses in combination with a dual-frame camera (e.g., the PCO SensiCam™ camera or the TSI PowerView™ camera) to capture two full resolution images in very quick succession. As is known in the art, a dual-frame camera uses a frame-transfer CCD array that includes an image region (i.e., the region which is sensitive to light for capturing an image) and a memory or storage region (i.e., the region to which a captured image is transferred for temporary storage prior to readout). Frame-transfer CCDs are widely used in cameras because they enable electronic shuttering and therefore eliminate the need for mechanical shutters. In the dual-frame mode of operation, the image region is exposed a first time whereby a first image is captured and transferred to the memory region for temporary storage and readout. Immediately after the first frame has been transferred to the memory region, the image region may be exposed a second time whereby a second image is captured and remains on the image region until the first image has been readout, whereupon the second image is transferred to the memory region for temporary storage and readout (i.e., the second image remains exposed on the image region for the entire readout time of the first image). The time needed to transfer a captured image from the image region to the memory region is very short (e.g., can be as low as 100 nanoseconds) compared to the readout time (e.g., tens or hundreds of milliseconds). Thus, each dual-frame camera (in conjunction with two short light pulses) can be used to capture two images in very quick succession (i.e., down to the image transfer time), but then has to wait tens or hundreds of milliseconds (i.e., the readout time of the two images) before it can capture another pair of images. The framing rates that may be achieved with this type of imaging system can be as high as 10 MHz, but the number of captured images in the sequence is limited to two.
In order to capture multiple full resolution images at higher framing rates, imaging systems have been used that employ some type of light source in combination with a multi-channel camera that incorporates multiple single or dual-frame cameras. In one such type of imaging system, a light source emits a single flash of light having a duration that is long enough to capture multiple images of a target. The light emanating from the target is then distributed to multiple cameras via the use of a specially designed beam splitter (or multiple 50/50 beam splitters arranged in a branching configuration) that splits the image into multiple identical images. The cameras are then triggered in quick succession so that each camera captures one image (for single-frame cameras) or two images (for dual-frame cameras) from the target. Framing rates of more than 100 MHz may be achieved with this type of imaging system. Examples of such multi-channel cameras are the Cordin-220™ camera, the Imacon 200™ camera, and the PCO HSFC-Pro™ camera.
With the multi-channel imaging system described above, the exposure time of each frame must be extremely short (e.g., in nanoseconds) so as to accommodate the high framing rate. As such, each camera requires the use of an image intensifier that acts as external shutter to control the exposure time of the camera. The image intensifier also amplifies the intensity of a received image in order to compensate for the short exposure time and the division of light by the beam splitter. The problem with the use of image intensifiers, however, is that they degrade the quality of the resulting images. Thus, this type of imaging system is not well-suited for techniques that require high quality images, such as digital image correlation and particle image velocimetry.
In another type of multi-channel imaging system, a light source is used to emit a single flash of light having a duration that is long enough to capture multiple images of a target. However, instead of using a beam splitter, the images from the target are distributed one at a time to multiple cameras via the use of a high-speed rotating mirror. While this type of imaging system does not require the use of image intensifiers, the framing rates are limited to a few megahertz because of the technical limitations imposed by the high-speed rotating mirror (e.g., a rotational speed of 4,000 revolutions per second is needed to capture images at a framing rate of 1 MHz) and the image blurring (known as image drag for this type of camera) caused by the rotation of the mirror. An example of a multi-channel camera of the rotating mirror type is the Cordin-510™ camera.
In yet another type of multi-channel imaging system, multiple nanosecond-pulsed lasers are used to emit multiple light pulses whereupon multiple images sequentially emanate from the target. The images are then distributed to multiple cameras (incorporating frame-transfer or interline CCDs) that are triggered in quick succession whereby each camera captures a single image of the target. This type of imaging system does not require the use of image intensifiers due to the ultra-short duration of the light pulses, which defines the effective exposure time of each camera. However, because the time between light pulses must be greater than the time required to transfer a captured image from the image region to the memory region, the framing rates that may be achieved with this type of imaging system cannot exceed 10 MHz (for cameras with the shortest frame transfer time of 100 nanoseconds given that the exposure time can be set to be 100 nanoseconds or less). An example of such a multi-channel camera is the LaVision SpeedStar-4™ camera, which uses four 1 microsecond exposure cameras to capture four images from a target at a maximum frame rate of 1 MHz. Note that if dual-frame cameras were used, the second image is unusable due to the long exposure time during which multiple exposures would occur.
Thus, there remains a need in the art for an imaging system that is capable of capturing multiple high resolution images at ultra-high framing rates. There is also a need in the art for an imaging system that is capable of capturing multiple images without the use of image intensifiers that cause degradation of image quality. There is a further need in the art for an imaging system that uses multiple dual-frame cameras to capture multiple images from a target, wherein both frames of each dual-frame camera are used for image capture.